The present invention relates to a method and apparatus for the detection of sound waves.
In systems for determining the direction of sound waves in water, it is known to form a sum signal and two difference signals to indicate the direction of impinging sound waves in a bearing indicator operating according to the sum/difference principle, and to employ a receiving arrangement constituted by a gradient ranging system having four omnidirectional piezoelectric hydrophones spaced from one another at a distance which is small compared to the wavelength of the sound waves to be received, the hydrophones being arranged in groups of two hydrophone pairs which are orthogonal to one another.
In order to indicate the direction of impinging sound waves through water according to the sum/difference principle, it is desirable to be able to form sum/difference signals also from sound waves of very low frequencies since a particularly high proportion of the sound energy produced by water vehicles is in the very low frequency range; this energy originates from the drive assembly and from the resonances generated in the water vehicles by the drive assembly.
Reference herein to a very low frequency range is intended to mean substantially a frequency range below 30 Hertz.
It is known to form the sum/difference signals from the signal voltages prouduced in the hydrophones of such a receiving arrangement via sum-and-difference amplifiers.
Thus German Pat. No. 1,566,858, and corresponding U.S. Pat. No. 3,496,527, issued to Gunter Ziehm et al on February 17th, 1970, disclose an arrangement of sum-and-difference amplifiers whose output signals are fed to a cathode-ray tube which constitutes the bearing indicator of the system.
German Pat. No. 1,566,837, and corresponding U.S. Pat. No. 3,464,056, issued to Gunter Ziehm et al on August 26th, 1969, disclose a similar arrangement in which the output signals of the sum-and-difference amplifiers are indicated after a correlating evaluation.
However, these arrangements have the drawback that they do not permit a ranging indication based on very low frequency sound waves through water.
The frequency response of the hydrophones has a lower limit, which is determined by the insulation resistance of the hydrophones, and at very low frequencies this resistance may already by small compared to the capacitive impedance of the hydrophones and forms a frequency dependent voltage divider. This phenomenon and an equivalent circuit of a hydrophone are described in a data book for designers by Brush Clevite, copyright 1966. Practical values of the capacitance C.sub.e resistance R.sub.c of a hydrophone are for instance: C.sub.e = 2nF.sup.e ; R.sub.c = 1 M .OMEGA. or 10 M .OMEGA., with these values, the lower frequency limit is 100 cps or 10 cps.
Due to this frequency dependence, phase errors occur in the formation of sum/difference signals for very low frequencies and such errors substantially falsify the direction indication, even if sound waves are received whose frequencies are higher by one order of magnitude than the frequency limit of the hydrophones.